The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11a (1999) and Draft IEEE Standard 802.11g (2002) High Rate PHY Supplements to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, has recently been commercialized with the promise of 54 Mbps effective bandwidth, making it a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
IEEE 802.11a and 802.11g or “802.11a/g” compliant transmission systems achieve their high data transmission rates through using Orthogonal Frequency Division Modulation or OFDM encoded symbols mapped up to 64 QAM multicarrier constellations and beyond. Generally, OFDM works generally by dividing one high-speed data carrier into multiple low speed sub-carriers which are used for transmission of data in parallel. Put another way, the data stream of interest is divided into multiple parallel bit streams, each transmitted over a different sub-carrier having a lower effective bit rate. Before final power amplification and transmission, the multicarrier OFDM symbol encoded symbols are converted into the time domain using Inverse Fast Fourier Transform techniques resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR). OFDM is also used in fixed broadband wireless access systems such as proposed in IEEE Standard 802.16a: Air Interface for Fixed Broadband Wireless Access Systems Part A: Systems between 2 and 1 GHz, Draft working document, February 2002, (“802.16a”) which is incorporated herein fully by reference.
In the case of 802.11a and 802.11g, there are up to 52 defined sub-carriers or tones, of which 48 are available to carry data (the 4 remaining are pilot sub-carriers or tones, which bear predetermined data). These sub-carriers are substantially orthogonal to each other, so they can be spaced closer together than in conventional frequency division multiplexing. Mathematically, the integral of the product of any two orthogonal sub-carriers is zero. This property allows the separating of sub-carriers at the receiver without interference from other sub-carriers.
Even where wireless communications leverage orthogonal sub-carrier techniques such as provided in 802.11a/g, they are still subject to environmental effects which can distort, disrupt or inject noise, thereby at least intermittently impact effective data throughput or other aspects of communications link quality or performance. Thus, planning and accommodating for foreseeable environmental effects is an important part of wireless communications system design, and providing a system that is responsive to such effects and self-heals or adapts link characteristics or operational parameters is desirable. In an 802.11a/g implementation, link characteristics such as data transmission rate, convolutional coding rate, packet/frame size, transmit power, receiver gain, and frequency selection can be altered to preserve the link in deteriorating conditions, as well as to upgrade effective data throughput in the link when conditions improve. To this end, it would be advantageous to provide a wireless communications system capable of assessing communications link quality or performance, and adapt link characteristics in accordance with such assessed quality or performance. Further, It would be advantageous if a measure of such link quality or performance can be accurately obtained in a computationally efficient manner, to provide a parameter useful for efficient link management and adaptation.